1. Field of the Invention
The present invention relates to a shadow mask structure for a cathode ray tube and, particularly, to a shadow mask structure for a cathode ray tube, which is provided with means for optomizing a relative positional relation between a face panel of glass and a shadow mask with respect to a variation of environmental temperature to restrict the mislanding.
2. Description of the Related Art
FIG. 5 is a partial cross section of a usual color cathode ray tube, showing a shadow mask structure mounted on an inner surface of a face panel of the tube.
As shown in FIG. 5, the shadow mask structure 1 comprises a shadow mask 2 having a number of fine through-holes 2a arranged in matrix for color selection, a frame 3 for reinforcing the shadow mask and hook springs 4 for mounting the shadow mask 2 inside a face panel 10 having a fluorescent film 11 formed thereon. The shadow mask 2 and the frame 3 are fixed to each other by such as welding in an overlapping portions 21 thereof. Each hook spring 4 takes in the form of metal piece of a material having the same temperature characteristics as that of the shadow mask and is fixed to an outer surface of the frame 3 by such as welding. The shadow mask structure 1 is fixedly mounted on the inner surface of the face panel 10 by fixedly fitting mounting holes 4a formed in end portions of the hook springs 4 on stud pins 20 provided on the inner surface of the face panel 10, respectively.
Now, the shadow mask structure 1 will be described.
FIG. 6 is a plan view of the shadow mask structure shown in FIG. 5 on the inner surface of the face panel when looked in a direction shown by arrows Y. As shown in FIG. 6, the shadow mask structure 1 is completed by partially overlapping a peripheral portion of the substantially rectangular metal frame 3 on a skirt portion 2b of the shadow mask 2, integrating the shadow mask 2 and the frame 3 by welding them at 8 points of a rectangular overlapped portion 21 thereof, 4 corners and intermediate points of sides of the overlapped rectangular peripheral portion and welding the hook springs 4 in the form of the metal pieces having the same temperature characteristics as that of the frame 3 to 3 points at both side wall portions 3a and a lower wall portion 3b. The end portions of the hook springs 4 are provided with the mounting holes 4a, respectively.
FIG. 7 is a partial cross section of the color cathode ray tube and shows a positional relation between the shadow mask 2 and a fluorescent film 11 formed on the face panel 10 thereof.
An electron beam 25 emitted from an electron gun (not shown) and deflected collides through the through-hole 2a of the shadow mask 2 with a film portion 11a of the fluorescent film 11 to excite the film portion 11a with a predetermined color. The frame 3 fixedly supporting the shadow mask 2 is fixedly supported by the hook springs 4 welded to the outer wall portion 3a of the frame 3 as shown in FIG. 5 and the stud pins 20 provided on the inner surface of the face panel 10.
In general, the shadow mask 2 and the frame 3 are made of iron containing materials. Therefore, their temperature is increased by bombardment of the electron beam 25 and the shadow mask 2 and the frame 3 are expanded to positions indicated in FIG. 7 by 2d and 3d, respectively.
Since the frame 3 is formed by using a plate material which is thicker than the shadow mask 2 and has larger thermal capacity, it expands thermally at low rate until thermal equilibrium is established. Thus, the through-hole 2a in the peripheral portion of the shadow mask 2 move outward to a position shown by 2e.
Therefore, the path of electron beam passing through the through-hole 2a to the predetermined film portion 11a of the fluorescent film 11 can not be established and the electron beam 25a passed through the through-hole 2e hits a film portion 11b remote from the predetermined film portion 11a. That is, the electron beam mislands, causing the color purity characteristics of the color cathode ray tube to be degraded.
Such thermal expansion of the shadow mask is called "whole doming". In a color cathode ray tube disclosed in Japanese Patent Publication (A2) No. Sho 49-9581, the whole doming phenomenon is relaxed by forming the hook springs from bimetal pieces to move the thermally expanded shadow mask toward the fluorescent film 11.
As also shown in FIG. 7, when the hook springs are formed from bimetal pieces, the position 3a of the frame 3 moves toward a position 3e and the position 2e of the through-hole is shifted to a position 2f. Therefore, an electron beam path passing through the through-hole 2f to the predetermined film portion 11a can be established.
In order to restrict the doming phenomenon, there is a current tendency of forming the shadow mask of an alloy such as invar which has a smaller thermal expansion coefficient than that of iron. Further, when the springs are of a spring steel such as SUS304 or SUS301, the doming phenomenon of the shadow mask is reduced to one third of that obtainable with the shadow mask of iron material.
In a color cathode ray tube such as mentioned above, the shadow mask 2 formed from invar material having small thermal expansion coefficient is free from thermal expansion due to environmental temperature variation. However, in a high definition display used in a terminal device, the thermal expansion phenomenon caused by environmental temperature variation of the face panel which is of glass having large thermal expansion coefficient compared with the shadow mask 2 becomes unable to be neglected and, when the display operates, heat is generated by a deflection yoke of the cathode ray tube and circuit portions of the display and transmitted upward by convection. Therefore, temperature in an upper portion of the display becomes higher than that in a lower portion thereof, which means that an upper portion of the face panel is easily expanded thermally compared with a lower portion thereof. Therefore, an amount of mislanding of electron beam in the upper portion of the face panel, that is, in an upper portion of a screen, becomes larger and, further, there is unbalance occurred in an image quality between the upper and lower portions of the screen.
FIGS. 8(a) and 8(b) are illustrations for explaining a principle of mislanding caused by environmental temperature variation of the color cathode ray tube. As shown in FIG. 8(a), at a certain constant temperature, the electron beam 25 passed through the through-hole 2a of the shadow mask 2 impinges the predetermined film portion 11a of a fluorescent film 11 on the inner surface of the face panel.
However, when the environmental temperature becomes higher than the certain constant temperature, the fluorescent film 11 is moved to a position 12 due to thermal expansion of the face panel 10 (not shown) and the predetermined film portion 11a moves the film portion 12a. Since the thermal expansion of the shadow mask 2 formed of invar material is very small, the electron beam 25 passed through the through-hole 2a impinges not the predetermined film portion 12a but the film portion 12b remote from the portion 12a.
On the contrary, when the environmental temperature becomes lower than the constant temperature, the fluorescent film 11 is moved to a position 13 due to thermal shrinkage of the face panel 10 (not shown) and thus the predetermined film portion 11 is moved to a position 13 and the predetermined film portion 11a is moved to a film portion 13a. Since the thermal shrinkage of the shadow mask 2 itself is very small and substantially negligible, the electron beam 25 passed through the through-hole 2a impinges not the desired film portion 13a but the film portion 13b remote from the film portion 13a.